In high power conversion field, a main control unit and a power module are isolated with each other by an optical fiber and a high voltage isolation power supply. For each power switch, the main control unit transmits a corresponding Pulse Width Modulation (PWM) signal to a gate driver circuit for the switch via an optical fiber. Meanwhile, the gate driver circuit sends a protection signal for the power switch to a low voltage control side via an optical fiber.
FIG. 1 is a schematic diagram showing a structure of a driving system for one phase (for example, Phase A) in a PWM Neutral Point Clamped (NPC) three-level converter. The system includes two main parts, i.e., a main control unit 11 and a power module 12. The main control unit 11, as the core control part of the system, is generally responsible for sampling of information such as system voltages and currents, implementation of control algorithms, generation of system timing, and generation of PWM switching signals. Usually, these functions can be realized by digital control chips which may be composed of one or a combination of a Digital Signal Processor (DSP), a Programmable Logic Controller (PLC), a Single Chip Microcomputer and even a Field-Programmable Gate Array (FPGA)/a Complex Programmable Logic Device (CPLD). The power module 12, as an executing mechanism in the system, is generally responsible for receiving the PWM switching signals transmitted from the main control unit 11, generation of corresponding switching actions, and power and energy conversion. In high power conversion field, when performing switching actions, the power module 12 usually generates large voltage jumps, thereby resulting in common-mode current interference. In order to prevent such interference from influencing the main control unit 11, isolations generated by magnetic cores or light are provided between the main control unit 11 and the power module 12. As shown in FIG. 1, magnetic isolation drivers for S1-S4 are used to achieve the isolation between the power module 12 and the main control unit 11. Also, respective ones of power switches in the power module 12 correspond to the isolation drivers one to one. For example, by the magnetic isolation driver 131 corresponding to S1, the magnetic isolation driver 132 corresponding to S2, the magnetic isolation driver 133 corresponding to S3, the magnetic isolation driver 134 corresponding to S4, the gate drivers of the power switches S1, S2, S3, and S4 in the power module 12 are isolated from each other without interference therebetween. Further, because the current flowing through the power module 12 is relatively large, and the power module 12 withstands relatively high voltage, it is needed to generate safe isolation between the power module 12 and the main control unit 11. Because of properties such as strong anti-interference capability and high insulation voltage, optical fibers are widely applied in the high power conversion field. Referring to FIG. 1, the DSP in the main control unit 11 transmits signals to respective magnetic isolation drivers via two optical fibers. For example, the main control unit 11 transmits PWM switching signals to the magnetic isolation driver 131 corresponding to S1 by a sending optical fiber Fiber1, and receives a failure protection signal for the power switch S1 via a receiving optical fiber Fiber2. Thus, a power module including four power switches need 4*2=8 optical fibers. For a three-phase NPC converter, a total of 8*3=24 optical fibers are needed. That is, if the conventional driving method for two-level or three-level converters is simply applied into a five-level, seven-level, or even nine-level converter, the number of needed optical fibers will rise greatly, and the driver circuit will become complicated.
With the development in the high power conversion field, increase of the number of levels in a converter can effectively improve electrical property of the converter. However, the increase of the number of levels will result in rise in number of the power switches, and a follow-on problem is that the needed sending and receiving optical fibers are increased. The increase in the number of optical fibers will increase costs, and system reliability will be greatly reduced due to high failure rate of the optical fibers.